The present invention relates to a hydrogen gas separator fixing structure and a hydrogen gas separating device using the same.
Hydrogen gas is used in a large amount as a basic raw material gas in petrochemical industry and also is expected highly as a clean energy source. In order to separate only a particular gas component from a mixed gas of multiple components, there has been known a separation method using an organic or inorganic, gas separation membrane, i.e. a membrane separation method. As the membrane used for hydrogen separation, there are known an organic polymer membrane such as polyimide, polysulfone or the like, a ceramic membrane such as silica or the like, and an inorganic compound membrane such as palladium, palladium alloy or the like. A palladium membrane or a palladium alloy membrane, in particular, is heat-resistant and further makes it possible to obtain hydrogen of extremely high purity.
Palladium or palladium alloy can absorb hydrogen alone, making possible selective separation of hydrogen. Palladium or palladium alloy is ordinarily used in the form of a thin membrane of about 1 to 20 μm in thickness, coated on a porous support of ceramic or the like, in order to achieve a high hydrogen permeation rate.
As to related conventional techniques, there are disclosed gas separators comprising a porous substrate and a gas separation membrane composed of palladium or palladium alloy, formed on one surface of the porous substrate (see, for example, Patent Documents 1 and 2). The porous substrate used therein is composed of glass, or a ceramic of aluminum oxide or the like. The gas separation membrane is formed on the porous substrate because no sufficient mechanical strength is obtained only with the gas separation membrane.
A gas separating device incorporating therein a gas separator such as mentioned above has such a structure that a gas separator is placed in a metal-made vessel, wherein a gas to be separated is introduced from one side of the gas separator, only a particular gas (hydrogen) permeates the gas separator, and purified gas (hydrogen) is taken out from the other side of the gas separator. Therefore, it is necessary that the gas to be treated side and the purified gas side are separated gas-tightly and the gas separator is gas-tightly connected to the purified gas outlet of metal-made vessel. Hence, it is important that the gas to be separated causes no leakage from the connection portion between the gas separator and the metal flange connecting to the purified gas outlet, to the purified gas side. Meanwhile, in order to efficiently separate hydrogen gas using the gas separator, the permeation rate of hydrogen atoms in gas separation membrane needs to be high; therefore, separation at a high temperature (300° or higher, preferably 500° or higher) and a high pressure (5 to 20 atm.) is advantageous. In this case, the gas tightness and durability of the connection part between the gas separator and the metal flange are important. That is, there is required such durability as to be able to sufficiently maintain the gas tightness under which the gas to be separated causes no leakage from the connection part between the gas separator and the metal flange to the purified gas side even under conditions of high temperature and high pressure.
In order to inhibit leakage of gas under the above conditions, it is generally conducted to braze the gas separator and the metal flange with a brazing material or the like (see, for example, Patent Document 3). It is also conducted to use an O-ring to secure the gas tightness between the gas separator and the metal flange therefor when the gas-treating temperature is 250° C. or lower.
In the above-mentioned brazing of the gas separator and the metal flange, however, there occur in some cases problems that the high temperature of brazing and resultant thermal stress causes the breakage of the porous substrate constituting the gas separator, or the load of heat cycle invites a reduction in gas tightness between the gas separator and the metal flange. Further, it is necessary to control the clearance between the gas separator and the metal flange therefor strictly and connect them in such a controlled state. Furthermore, the high brazing temperature incur in some cases, problems such as the breakage due to melting and the strain generation due to heat stress, of the gas separation membrane constituting the gas separator. In using the O-ring, it has been substantially difficult to maintain sufficient gas tightness when the temperature of gas treatment is higher than 250° C., and the there has been a limitation in applicable temperature range.
In order to solve the above-mentioned problems, there are disclosed a gas separator fixing structure wherein a gas separator and a metal flange are sealed with a gland packing (see, for example, Patent Document 4). Since this gas separator fixing structure employs no brazing, there hardly occurs a problem of breakage of substrate due to thermal stress; however, gas-tight brazing completely free from leakage is difficult to achieve and a countermeasure for leakage from connection portion is necessary. In applications where separation of higher-purity gas is required, there has been necessary even higher gas tightness between gas separator and metal flange therefor.                Patent Document 1: JP-A-1987-273030        Patent Document 2: JP-A-1988-171617        Patent Document 3: JP No. 3305484        Patent Document 4: JP-A-2003-126662        